Conventional postage stamps and revenue stamps (hereinafter referred to as "stamps" collectively) have on their reverse side an adhesive layer formed by coating a water-soluble polymer such as polyvinyl alcohol. The adhesive layer permits used stamps to be easily peeled off from envelopes, postcards, etc. by stamp collectors when dipped in water. However, this advantage is offset by a disadvantage that the adhesive layer has to be moistened when stamps are stuck to envelopes and postcards. Moistening needs water, but water is not always available, and water is often substituted by saliva. Licking stamps is unsanitary and indecent. In addition, moistening many stamps is a time-consuming work and incomplete moistening results in poor adhesion.
In order to eliminate the above-mentioned disadvantage, there was proposed a stamp of the self-adhesive paper type which can be stuck simply and efficiently. (See Japanese Utility Model Laid-open Nos. 77700/1977 and 108167/1981.) This stamp has a structure as shown in FIG. 1 and it comprises a release member 1, a pressure sensitive adhesive layer 2, a water-soluble polymer layer 3, and a stamp member 4. When the release member is removed, the stamp member is ready to be stuck. In actual practice, however, the stamp member of this structure was found to have a disadvantage that the stuck stamp member easily peels off even though it is not dipped in water, because there is not sufficient bond strength between the pressure sensitive adhesive layer 2 and the water-soluble polymer layer 3. Therefore, the stamp of this structure is not of practical use, unless this problem is solved.
On the other hand, conventional self-adhesive labels, seals, stickers, slips, etc., which do not have any water-soluble polymer layer as in the case shown in FIG. 1, can be stuck without the need of being moistened, and they do not peel off easily once they are stuck. Their disadvantage is that they cannot be easily peeled off by dipping in water when stamp collectors want to peel them off.